1. Field of the Invention
This invention relates to a catalyst for use in auto emission control which has improved, lower H.sub.2 S emissions while serving as a three-way catalyst (TWC).
2. Description of the Previously Published Art
With the arrival of the new "High Tech" auto exhaust catalysts coupled with the new operating conditions of the current state-of-the-art engine air/fuel management systems, there has resulted a new emission problem. That problem emission is H.sub.2 S. It is responsible for the rotten egg odor which occurs after certain modes of operation (e.g., sudden braking and quick starting). Additives such as nickel have been added to conventional catalyst compositions which consist typically of platinum group metals supported on a rare earth oxide promoted alumina. Such rare earth oxide and Ni compositions are currently in commercial use. While nickel is effective in lowering H.sub.2 S emissions, it is a suspected human carcinogen. Thus nickel may at some time be restricted in its use because of the potential threat to the environment as well as to public health. Some West European countries have not recommended its use. My approach is to formulate catalysts with additives which do not pose such a threat to the public health and the environment.
While rare earths and especially cerium have been used commercially in auto emission control catalysts for a number of years, these catalysts when used in conjunction with the new closed loop air/fuel management systems have been found to produce significant H.sub.2 S emissions. This is most likely a result of ceria's efficacy for storing sulfur and for generating hydrogen via the water gas shift reaction under fuel rich (O.sub.2 deficient) conditions the latter having been pointed out by G. Kim in "Ceria Promoted Three-Way Catalysts for Auto Exhaust Emission Control", I&EC Product Research & Development, 1982, 21, 267. These H.sub.2 S emissions have reached levels that the consumers feel are objectionable.
All of the more common rare earths (viz., La, Ce, Nd, and Pr) to varying degree when incorporated into auto exhaust catalysts generate undesirable levels of H.sub.2 S. Catalysts lacking cerium oxide as a major component generally do not have sufficient activity to catalytically remove carbon monoxide under the conditions which the current state-of-the-art engines operate. That is the reason that higher levels of cerium oxide and other rare earth oxides have been incorporated into the so-called "High Tech" catalysts. "High Tech" performance has led to increased incidence of abnormally high levels of H.sub.2 S emission. My desire has been to achieve satisfactory carbon monoxide removal, but without the creation of new pollution problems.
3. Objects of the Invention
It is an object of this invention to obtain a catalytic composition which produces low H.sub.2 S emissions and is capable of meeting the EPA requirements for CO, HC, and NOx emissions.
It is further object of this invention to achieve good CO and HC conversions under fuel rich, oxygen deficient conditions, with minimal H.sub.2 S emissions.
It is further object of this invention to obtain a catalytic composition which is relatively non-toxic, and which does not pose potential threats to public health and the environment as current nickel containing catalysts may do.
It is further object of this invention to obtain a catalyst suitable for use as a three-way catalyst having low H.sub.2 S emissions for emission control which has substantially no ceria present and which has a formed refractory oxide particle or refractory oxide powder support which has deposited thereon a non-rare earth oxide stabilizer, an alkali metal oxide promoter, and one or more platinum group metals as the catalyst metals.
It is further object of this invention to make a catalyst which is suitable for use as a three-way catalyst with low H.sub.2 S emissions for emission control by impregnating a refractory oxide support with a solution containing either a non-rare earth stabilizer salt or a non-rare earth stabilizer salt plus an alkali metal compound; heating the impregnated support to at least decompose the non-rare earth stabilizer salt; and applying one or more catalytic platinum group metals and optionally at least one alkali metal promoter.
It is further object of this invention to treat exhaust gases so as to reduce H.sub.2 S emissions by passing the exhaust gases over a catalyst suitable for use as a three-way catalyst having low H.sub.2 S emissions for emission control which has substantially no ceria present according to this invention.
These and further objects will become apparent as the description of the invention proceeds.